Thanks for adding the Internationalization Considerations section, which addresses the concern in my previous review that this issue needed more work.  I have small issues:

1. There is a reference to "control characters" without any citation, and the definition is not as obvious as one might think. A good reference is [UNICODE] section 23.1, which may be summarized as "65 code points in the ranges U+0000-U+001F ("C0 Controls") and U+0080-U+009F (“C1 Controls”), plus U+007F, "DEL"
2. This section might benefit from a reference to RFC9413, which has a thorough discussion of dealing with input data that is invalid for some reason.  
3. There are 3 options when receiving a message with invalid characters: delete them, replace them (Unicode provides U+FFFD for this purpose), or reject the message. The draft mentions two of these but not the replacement option. Is it never appropriate in the JMAP context?